Leukocyte activation and adhesion to the endothelium are pivotal steps in the recruitment of cells to the inflamed tissue. This coordinated sequence of adhesive events on the endothelium includes the rolling, the leukocyte activation, the firm adhesion to and subsequent locomotion and the trans-endothelial migration. During trans-endothelial migration, molecules located at the interendothelial junctions may facilitate the passage of the transmigrating leukocyte. We have recently identified the third member of the JAM family, JAM-C, and could demonstrate that JAM-C is expressed at the interendothelial junctions and serves as a counter-receptor for neutrophil Mac-1, mediating the transendothelial migration of the neutrophils in vitro and in vivo. In contrast, JAM-C does not participate in the adhesion of leukocytes to quiescent endothelial cells, as it is located at the interendothelial junctions and is not available for leukocyte integrin Mac-1. In contrast to quiescent endothelial cells, where JAM-C localized strictly to cell-cell contacts and colocalized with a tight junction component, ZO-1, JAM-C localization on oxLDL stimulated endothelial cells was no more restricted to intercellular contacts. In particular, on oxLDL-pretreated HUVEC a part of JAM-C re-distributed to sites distinct of the interendothelial contacts (at the apical surface). In this case JAM-C on endothelial cells may also function to mediate Mac-1-dependent adhesion of leukocytes. Thus, JAM-C participates in the oxLDL-mediated upregulation of leukocyte extravasation with potential implications for the enhanced inflammatory cell recruitment to the atherosclerotic vessel wall. The aim of our current studies is:a) To study the molecular mechanisms of JAM-C mediated neutrophil transendothelial migration. In particular, how is JAM-C distributed in the interendothelial contacts during neutrophil transmigration?What particular step of transendothelial migration is mediated by JAM-C? In addition, the importance of JAM-C in leukocyte transendothelial migration in vivo will be tested with the use of JAM-C -/- mice, which will be soon available in our lab.b) The role of JAM-C in endothelial paracellular permeability. As JAM-C is loacated at the interendothelial tight junctions, it may regulate permeability of the monolayer, and permeability changes are crucial during inflammatory reactions and in angiogenesis. We are trying to understand whether JAM-C is recruited to the junctions to regulate permeability. Are these JAM-C-mediated changes due to alterations of the actin cytoskelton and of endothelial contractility? Does the presence of JAM-C in tight junctions affect the integrity junctions?3) The homophilic interaction of JAM-C and its functional consequences. We could biochemically characterize the homophilic interaction of JAM-C and identify the amino acid sequence responsible for this interaction. In an effort to study the functional significance of this interaction, we identified that this interaction may participate in adhesive interactions of tumor cells that express JAM-C (e.g. we found non-small cell carcinoma cells and melanoma cells to express JAM-C) with the endothelium. Thus, JAM-C may play a role in tumor metastasis.